The invention concerns methods and devices for monitoring and correcting a sensorless rotor position detection in permanently excited motors with a control device and current converter.
Permanently excited synchronous motors (PMSM) and brushless direct current motors (BLDC) are comprised of a stator and a rotor with magnets. The individual phases of the stator are star-connected or delta-connected. The motor is operated with a current converter. These motors require electronics that detects the rotor position and supplies the individual phases with current.
The rotor position can be detected by means of sensors. Inter alia, Hall sensors are employed. Sensors require mounting space and cause higher system costs.
In order to avoid these disadvantages, sensorless control methods are employed. They can be roughly classified in two groups.
One group employs the voltage that is induced by movement in the phases. The system-based disadvantage lies in the lack of positional information at standstill.
The second group is based on the variation of the stator inductances. The saturation of the stator inductance is affected by the rotor field of the permanent magnets and the current-caused stator field.
The inductance of a coil in the currentless state is proportional to the relative permeability. As a result of the saturation effects that occur in the core, with increasing field strength the relative permeability drops or, stated differently, the magnetic resistance increases. In this connection, only the absolute value but not the direction is decisive. Thus, a rotor position-dependent function of the inductance for a BLDC motor results. When the motor has three phases, the functions are displaced relative to each other by 60 electrical degrees. In the currentless state, the north and the south poles of the rotor magnets have the same effect. Therefore, these functions of the inductivities have twice the periodicity relative to the electrical period. This ambiguity must be resolved for a complete position information. A current-excited field is superimposed on the stator field. In this way, the saturation in the motor is either amplified or reduced and the corresponding inductance is thus reduced or enlarged. This effect affects also the angle determination in case of useful current feed of the motor and causes a faulty determination of the rotor position.
The invention has the object to simply resolve the ambiguity of the rotor position, determined based on the inductance ratios of the motor, for permanently excited motors without a sensor and, as needed, to correct an erroneously determined angle.